The drill bit for drilling oil and gas wells is facilitated by having a heavy load applied to assist in crushing and pulverizing the formation being drilled. The formation material must be reduced to particles small enough that the flow of drilling mud up to the surface will carry it to the surface. Drill collars are connected to the drill bit to provide the heavy load for this purpose.
The drill bit and drill collars are part of a drill string which also includes drill pipe which extends to the drilling rig at the surface.
The drill pipe which extends to the surface is thin walled. Its primary design requirement is to support the weight of the drill string including the drill collars during running and retrieving of the drill string.
Conversely, the drill collars are at the bottom of the drill string and they only support themselves. The drill pipe can be 20,000 feet long or longer and drill collars seldom exceed 1,000 feet in length. Although the drill collars are heavier, there is much more length in drill pipe, and the drill pipe must support the drill collars and the drill pipe.
Drill collars have as small a bore as practical and as large an outer diameter as is practical so that they will be heavy. The drill collars have metal sealing threaded connections on each end. These threaded connections are benefited by being made of high strength steel. As a result the entire drill collar is made of high strength steel. They are extremely strong as a result, but do not have a requirement for being extremely strong. They are characteristically so strong that the average person presumes they need to be strong, because they always are.
A problem resulting from this is that the thick cross section of high strength steel cannot be sheared by the blind shear rams in the primary well control device, the blowout preventer stack. The blind shear rams are to cut the pipe in the bore and seal across the bore to keep a well from blowing out. When as much as 1000 feet of drill collars pass in front of the blind shear rams, the well bore literally cannot be closed.
On land or platform wells this is not a major concern as in unexpected pressure situations there is always a closable valve on the top of the drill string except for the short time for making connections at the surface. For the annular area between the outside diameter of the drill string in the well and the bore of the blowout preventer stack, there are annular and ram type blowout preventers which are well known in the art and can be closed to seal this annular area.
In deepwater drilling situations from a floating vessel the situation is different. In the worst case scenario the vessel can be blown off location or can have a steering computer accidental drive off when you are in an unexpected pressure situation. If this happens when the drill collars are in the bore in front of the blind shear rams, you cannot close the blowout preventers and you cannot let go of the pipe string. In other words you have a blowout.